1. Field of the Invention
The present invention relates generally to bioreactors for growing cultures. More particularly, the present invention relates to a bioreactor employing spin filter perfusion technology.
2. Related Art
Traditional methodologies for culture of suspension cells include batch culture, fed batch culture and chemostat culture. All of these methods do not allow for concentration of cells within the culture vessel. In these systems, maximal cell yield is limited by the fact that nutrients in the growth medium become depleted and waste products increase to deleterious levels. For many suspension cell types, this allows for growth slightly above 10.sup.6 cells/mL. Suspension cells that produce a biological product often demonstrate maximum production when the cells have achieved their peak density. Unfortunately, at this point, many of the nutrients necessary for production of the biological are already depleted.
Replacing the supernatant with fresh medium while containing the cells within the bioreactor would be advantageous not only for cell growth but also for yield of the biological product. This would also allow for an efficient continuous culture that could be maintained over long periods of time with lower cell turnover rates as compared to chemostat culture. By keeping a low cell turnover rate, the selection pressures should be much reduced and energy and materials could be turned toward production of product as opposed to cell growth.
Such cell concentration is possible using several methodologies to include hollow fiber cell concentration, external centrifuges, and spin filter perfusion technology.
Hollow fiber units can be used to concentrate cells in a culture while perfusing nutrients through the culture. Cells and media pass through the inner capillary spaces where spent culture media (but not cells) pass across the fiber to the extra-capillary space. Cells are inhibited by passing due to fiber pore size. Fresh medium is then added to the bioreactor elsewhere. However, change to a different cell type results in need to recalibrate perfusate/retentate flow rates to prevent plugging. Also, pump shear forces on the cell culture passing through the hollow fiber may be significant since flow rate must be significant to prevent cells becoming trapped against the sides of the inner fiber wall.
Use of an external centrifuge to concentrate cells in a culture has been advocated but has practical drawbacks such as expense and complexity of design.
Biospin cell concentration as a means of perfusing cultures has been advocated for several years. Biospin cell concentration is described generally in Spin Filter Culture: The propagation of Mammalian Cells in Suspension, Himmelfarb et al., Science 164: 555-557, 1969). This technology generally consists of a mesh filter spinning around a tube. The spinning of the filter causes spent culture medium to pass through the filter and into the tube where it is removed to an external collection vessel.
Conventional spin filter units, however, have several significant disadvantages. Conventional spin filter units do not provide sealing as the filter rotates about the tube. Having no seal results in cell culture entering the tube during spinning and/or the introduction of air into the tube which thereby limits the perfusion rate. Limiting the perfusion rate significantly affects cell yield due to not being able to perfuse enough cell culture medium to satisfy a high density culture.
Further, conventional spin filter units have a relatively complex internal structure, making capacity scaleup or scale-down, assembly and disassembly, and sterilization difficult. Scale-up and or scale-down in particular, is a very desirable feature for perfusion processing operations.